(1) Field of the Invention
The present invention relates to a sensor for measuring the air-fuel ratio of gases to be measured, such as exhaust gas and the like (hereinafter, gas to be measured may be merely referred to as gas) from the oxygen concentration in these gases by measuring the electrical resistance of the sensor, and a method of measuring the air-fuel ratio by using said sensor.
(2) Description of the Prior Art
There has been proposed to drive a motorcar at a lean side, wherein the air-fuel ratio of an intake mixture is higher than the theoretical air-fuel ratio (excess air ratio .lambda.=1) in order to lower the fuel consumption of motorcar engine and to purify the exhaust gas, and such a sensor is demanded that can measure accurately the air-fuel ratio within the range of the excess air ratio .lambda. of not less than 1. Japanese Patent Laid Open Application No. 62,349/80 discloses a method, wherein a direct current having a value lower than the critical electric current value, at which the electromotive force does not vary against the air-fuel ratio other than the theoretical air-fuel ratio, is flowed between the two electrodes in an oxygen concentration cell comprising a measuring-side electrode layer, an oxygen ion-conductive solid electrolyte layer, a reference-side electrode layer and a membrane layer superposed one upon another, whereby the air-fuel ratio is measured by the electromotive force caused between the above described electrodes. However, in this method for measuring air-fuel ratio, the relation between the excess air ratio .lambda. and the above described electromotive force is not linear in the region of almost .lambda.=1 within the range of the excess air ratio .lambda. of not less than 1. Therefore, this method is disadvantageous in the measuring of air-fuel ratio. Moreover, the sensor disclosed in the Japanese patent application has a drawback that, when it is intended to arrange the sensor in a plug and to use the plug as a plug for measuring air-fuel ratio, the structure of the plug is complicated.